For example, from British patent specification No. 819,549 such a lamp shade is known, in which two separate side shade plates defining the side shade-faces are arranged with an upper mutual distance of more than 2 d. The said shade plates are at their upper edges provided with inwardly extending, downwardly bowed retaining tongues adapted to rest on the tubular lamp. Each side shade-face has in cross sections an upper outwardly extending approximately plane portion that through a bow continues into a downwardly extending part following such a slightly curved curve that the said parts of the shade-faces diverge over their whole height. The shade is relatively shallow so that when the shade is arranged on a tubular lamp a plane through the lower edges of the side shade-faces will be situated at a distance from the lamp of the same order of magnitude as d.
Such a shade provides an acceptable protection against glare in its transverse direction as well as in its longitudinal direction, the latter owing to the cross-plates, but it permits only utilizing of theoretically at a maximum 70% of the light emitted from the tubular lamp, and in practice much less than that. Further the direct downwardly emitted light from the lamp together with the light reflected from the lamp shade provides a highly irregular dispersion of the light, resulting in a relatively powerfully illuminated zone directly below the shade and relatively wide essentially more badly lit border zones.
Somewhat the same is the case in connection with the lamp shade known for example from the German published patent application No. 2,410,037 and also adapted for being arranged directly on a tubular lamp. This known shade comprises a half cylindrical retaining plate embracing the upper half of the lamp from which retaining plate the real shade plates defining the side shade-faces extend. The cross sections of these side shade-faces follows a parabola, and the plane through the lower edges of the side shade-faces is situated at a relatively short distance from the lamp of between d and 1.5 d. Owing to the retaining plate, such a shade permits only utilizing less than 50% of the light emitted from the tubular lamp. Since this known shade is not provided with cross-plates it does not protect against glare in the longitudinal direction of the tubular lamp.